Principios básicos y beneficios de los sistemas pasivos

The basic principles of passive systems are grounded in harnessing natural energy flows — solar radiation, wind, ground temperature, and daily thermal cycles — to maintain indoor comfort conditions without relying on mechanical heating or cooling systems, or with minimal dependence on them. The concept was formally born from Baruch Givoni's research in the 1960s and was consolidated with the Passivhaus standard, developed by Wolfgang Feist at the Passivhaus Institut in Darmstadt (Germany) in 1991. As of 2024, there are more than 65,000 Passivhaus-certified buildings worldwide, distributed across 90 countries with a cumulative area exceeding 50 million m² (Passivhaus Institut, 2024). These buildings achieve heating demands equal to or below 15 kWh/m²·year, representing a reduction of 75% to 90% compared to the 60-150 kWh/m²·year that characterize the pre-2006 European building stock.

Passive systems operate through five interdependent physical mechanisms: solar gain through south-facing glazed surfaces (in the Northern Hemisphere), thermal storage in mass elements (walls, slabs, floors with densities of 1,800-2,400 kg/m³), energy conservation through insulation with U-values below 0.15 W/m²·K for walls and roofs, heat distribution by natural convection and radiation between surfaces, and regulation of exchanges with the outdoors through controlled ventilation and solar shading. The essential difference from active systems lies in the energy source used: while a conventional heating system consumes between 80 and 200 kWh/m²·year of primary energy, a well-designed passive system can maintain indoor temperatures between 20°C and 26°C for 90-95% of annual hours without external energy input, leveraging internal gains (occupants, appliances: 2-5 W/m²) and passive solar gains (10-40 kWh/m²·year on the south facade with low-emissivity glass).

The thermal envelope is the most decisive principle of passive systems. An exterior wall with 25-40 cm of insulation (mineral wool, EPS, wood fiber, or blown cellulose) achieves U-values of 0.10 to 0.15 W/m²·K, reducing transmission losses to 3-5 W/m² when the indoor-outdoor temperature difference is 30°C. Roofs require even more demanding insulation (30-50 cm, U ≤ 0.10 W/m²·K) as they are the surface with the greatest exposure to nighttime radiation. Windows, historically the weak point of the envelope, have evolved to achieve U-values of 0.70-0.80 W/m²·K with triple low-emissivity glazing filled with argon or krypton and PVC frames with 5-7 chambers or wood-aluminum frames with thermal breaks. Treatment of thermal bridges — wall-slab junctions, window perimeters, corners — is critical: a linear thermal bridge of 0.10 W/m·K along a slab perimeter of 40 m generates losses equivalent to 4 W/K, comparable to those of the wall itself in highly insulated buildings.

Air tightness, measured by the Blower Door test in accordance with standard EN 13829 (superseded by ISO 9972), is the second fundamental principle. The Passivhaus standard requires an air change rate at 50 Pa (n₅₀) equal to or below 0.6 air changes/hour, while a conventional Spanish building typically shows values of 4 to 10 ACH. Uncontrolled air infiltration accounts for 25% to 40% of total heat losses in poorly sealed buildings (Feist, 2013). Achieving the required airtightness demands a continuous air barrier — typically a 0.2 mm polyethylene membrane or a 15 mm interior plaster coat — with all joints treated using certified adhesive tapes (adhesion ≥ 600 N/m per the FIW standard) and sealing of the 200-400 typical penetrations in a single-family dwelling (ducts, cables, pipes). Mechanical ventilation with high-efficiency heat recovery (85-95% sensible heat recovery) replaces accidental infiltration with a filtered, preheated, and controlled airflow at rates of 30 m³/h per person according to standard EN 16798-1.

Passive solar gain transforms a building's south facade into a natural thermal collector. At latitudes of 36° to 43°N (mainland Spain), a south-facing glazed surface receives between 3.5 and 5.0 kWh/m²·day of solar radiation in winter on the vertical plane, while in summer the irradiance drops to 1.5-2.5 kWh/m²·day due to the high solar altitude (angle of 70-75° at noon in June). This seasonal asymmetry allows designing overhangs of 60-100 cm that block 80-100% of direct solar radiation in summer without obstructing winter gain. A classic Trombe wall — exterior glass, 5-10 cm air gap, 20-40 cm concrete wall painted dark (absorptance ≥ 0.90) — captures between 150 and 300 kWh/m²·year of solar energy and transfers 40 to 100 kWh/m²·year to the interior, with a thermal lag of 6 to 12 hours that shifts solar gains from midday to nighttime hours.

Thermal inertia, provided by materials of high density and high specific heat, acts as a buffer against thermal oscillations. A 20 cm reinforced concrete slab stores 46 Wh/m²·K, enough to absorb daytime solar and internal gains and release them during the night. In climates with daily thermal oscillations greater than 15°C — common in the continental zones of the Castilian plateau — the combination of thermal inertia and nighttime ventilation reduces the maximum indoor temperature by 5°C to 10°C compared to the outdoor peak (Givoni, 1994). Natural cross ventilation, generated by pressure differences between opposite facades (1-10 Pa with winds of 1-5 m/s), achieves flow rates of 10 to 30 air changes/hour with openings of 5-10% of the facade area, sufficient to dissipate heat loads of 20-40 W/m². Stack-effect ventilation harnesses the density difference of warm air (rising) to generate flow rates of 3-8 ACH with indoor-outdoor temperature differences of 3-6°C and stack heights of 3-6 m.

The benefits of passive systems are documented across three dimensions with data from more than 1,000 monitored buildings in Europe. In the energy dimension, Passivhaus buildings consume 80% to 90% less energy for heating than conventional buildings of the same construction period: monitoring from the CEPHEUS project (Cost Efficient Passive Houses as European Standards, 2001) across 221 dwellings in five European countries confirmed average heating demands of 14.9 kWh/m²·year, within the Passivhaus limit. Total primary energy consumption (heating, cooling, DHW, electricity) stands at 80-120 kWh/m²·year, compared to 200-350 kWh/m²·year for conventional buildings. In the economic dimension, the construction premium ranges from 5% to 15% depending on the local market and the technical team's experience (Passivhaus Institut, 2015): in Germany, the average premium has fallen from 16% in 2000 to 3-8% in 2020 as the supply chain has matured.

The payback period for the construction premium ranges from 7 to 12 years considering energy savings alone, and drops to 4-8 years when lower HVAC maintenance requirements are included (absence or reduction of boilers, split units, ductwork). In the health and comfort dimension, indoor air quality measurements in Passivhaus dwellings show CO₂ concentrations consistently below 1,000 ppm (average: 700-850 ppm) thanks to continuous mechanical ventilation, compared to peaks of 2,000-4,000 ppm common in bedrooms of dwellings without mechanical ventilation. Ventilation air filtration (filters F7/ePM1 55%) reduces PM2.5 concentrations by 60-80% and pollen by 95-99%, with documented benefits for people with asthma and allergies (symptom reduction of 40-60% according to Swedish studies from the Karolinska Institutet). The interior wall surface temperature in Passivhaus buildings remains above 17°C even with outdoor temperatures of -10°C, eliminating the sensation of cold radiation and the risk of surface condensation that causes mold.